For over three decades, battery scientists have been frustrated with the low energy density of lithium-ion cells primarily due to the low lithium-storing capacity of all existing cathode active materials. Specifically, the practical specific capacity achievable with current cathode materials has been limited to the range of 150-250 mAh/g, mostly less than 200 mAh/g. Although several high-capacity anode active materials have been found (e.g., Si with a theoretical capacity of 4,200 mAh/g), there has been no corresponding high-capacity cathode material available.
Further, most of the commercially available Li-ion cells make use of carbon- or graphite-based anodes, which have several significant drawbacks: low specific capacity (theoretical capacity of only 372 mAh/g of graphite), slow Li intercalation (due to low solid-state diffusion coefficients of Li in graphite) resulting in a long recharge time, inability to deliver high pulse power, and necessity to use lithiated cathodes (e.g. lithium cobalt oxide), thereby limiting the choice of available cathode materials. Furthermore, these commonly used cathodes also rely upon extremely slow Li diffusion in the solid state. These factors have contributed to the two major shortcomings of today's Li-ion batteries—a low energy density (typically 120-180 Wh/kgcell) and low power density (˜0.5 kW/kgcell).
Most recently, we proceeded to go beyond the mindset of using graphitic or carbonaceous materials either as an anode active material or as a cathode supporting material (i.e. as a conductive additive for the cathode). We have demonstrated the feasibility of implementing these materials as a cathode active material, responsible for storing lithium ions when the Li-ion cell is discharged to supply electricity to an external load. This is of great scientific and technological significance at least for two reasons. First, the commonly used cathode materials, such as lithium cobalt oxide and lithium iron phosphate, have relatively low specific capacities and, hence, a strong need exists for a higher-capacity cathode. Second, it is commonly understood in the battery industry and the field of electrochemistry that if a material is a good anode active material for a lithium-ion cell, the same material is usually not considered to be a viable cathode active material for a lithium-ion cell from the electrochemical potential perspective. We have defied this expectation with the discovery that a range of graphitic and carbonaceous materials can be used as a high-capacity and high-power cathode active material in a surface-mediated cell or SMC [Ref 1-7 below].
The following list of references is herein cited as part of the background information:    1. C. G. Liu, et al., “Lithium Super-battery with a Functionalized Nano Graphene Cathode,” U.S. patent application Ser. No. 12/806,679 (Aug. 19, 2010).    2. C. G. Liu, et al, “Lithium Super-battery with a Functionalized Disordered Carbon Cathode,” U.S. patent application Ser. No. 12/924,211 (Dec. 23, 2010).    3. Aruna Zhamu, C. G. Liu, David Neff, and Bor Z. Jang, “Surface-Controlled Lithium Ion Exchanging Energy Storage Device,” U.S. patent application Ser. No. 12/928,927 (Dec. 23, 2010).    4. Aruna Zhamu, C. G. Liu, David Neff, Z. Yu, and Bor Z. Jang, “Partially and Fully Surface-Enabled Metal Ion-Exchanging Battery Device,” U.S. patent application Ser. No. 12/930,294 (Jan. 3, 2011).    5. Aruna Zhamu, Chen-guang Liu, X. Q. Wang, and Bor Z. Jang, “Surface-Mediated Lithium Ion-Exchanging Energy Storage Device,” U.S. patent application Ser. No. 13/199,450 (Aug. 30, 2011).    6. Aruna Zhamu, Chen-guang Liu, and Bor Z. Jang, “Partially Surface-Mediated Lithium Ion-Exchanging Cells and Method of Operating Same,” U.S. patent application Ser. No. 13/199,713 (Sep. 7, 2011).    7. Bor Z. Jang, C. G. Liu, D. Neff, Z. Yu, Ming C. Wang, W. Xiong, and A. Zhamu, “Graphene Surface-Enabled Lithium Ion-Exchanging Cells: Next-Generation High-Power Energy Storage Devices,” Nano Letters, 2011, 11 (9), pp 3785-3791.
There are two types of SMCs: partially surface-mediated cells (p-SMC, also referred to as lithium super-batteries) and fully surface-mediated cells (f-SMC). Both types of SMCs contain the following components: (a) an anode containing an anode current collector (such as copper foil) in a p-SMC, or an anode current collector plus an anode active material in an f-SMC; (b) a cathode containing a cathode current collector and a cathode active material (e.g. graphene or disordered carbon) having a high specific surface area; (c) a porous separator separating the anode and the cathode, soaked with an electrolyte (preferably liquid or gel electrolyte); and (d) a lithium source disposed in an anode or a cathode (or both) and in direct contact with the electrolyte.
In a fully surface-mediated cell, f-SMC, as illustrated in FIG. 2(A) to FIG. 2(C), both the cathode active material and the anode active material are porous, having large amounts of graphene surfaces in direct contact with liquid electrolyte. These electrolyte-wetted surfaces are ready to interact with nearby lithium ions dissolved therein, enabling fast and direct attachment of lithium ions onto graphene surfaces and/or a redox reaction between a lithium ion and a surface functional group, thereby removing the need for solid-state diffusion or intercalation. When the SMC cell is made, particles or foil of lithium metal are implemented at the anode (FIG. 2(A)), which are ionized during the first discharge cycle, supplying a large amount of lithium ions. These ions migrate to the nano-structured cathode through liquid electrolyte, entering the pores and reaching the surfaces in the interior of the cathode without having to undergo solid-state intercalation (FIG. 2(B)). When the cell is re-charged, a massive flux of lithium ions are quickly released from the large amounts of cathode surfaces, migrating into the anode zone. The large surface areas of the nano-structured anode enable concurrent and high-rate deposition of lithium ions (FIG. 2(C)), re-establishing an electrochemical potential difference between the lithium-decorated anode and the cathode.
In a p-SMC, the anode comprises a current collector and a lithium foil alone (as a lithium source), without an anode active material to capture and store lithium ions/atoms. Lithium has to deposit onto the front surface of an anode current collector alone (e.g. copper foil) when the battery is re-charged.
The features and advantages of SMCs that differentiate the SMC from conventional lithium-ion batteries (LIB), supercapacitors, and lithium-ion capacitors (LIC) are summarized below:                (A) In an SMC, lithium ions are exchanged between anode surfaces and cathode surfaces, instead of the bulk or interior of an electrode active material:                    a. The conventional LIB stores lithium in the interior of an anode active material (e.g. graphite particles) when the LIB is in a charged state (e.g. FIG. 1(C)) and the interior of a cathode active material in a discharged state (FIG. 1(D)). During the discharge and charge cycles of a LIB, lithium ions must diffuse into and out of the bulk of a cathode active material, such as lithium cobalt oxide (LiCoO2) and lithium iron phosphate (LiFePO4). Lithium ions must also diffuse in and out of the inter-planar spaces in a graphite crystal serving as an anode active material. The lithium insertion or extraction procedures at both the cathode and the anode are very slow, resulting in a low power density and requiring a long re-charge time.            b. When in a charged state, a symmetric supercapacitor (EDLC) stores their cations near a surface (but not at the surface) of an anode active material (e.g. activated carbon, AC) and stores their counter-ions near a surface (but not at the surface) of a cathode active material (e.g., AC), as illustrated in FIG. 1(A). When the EDLC is discharged, both the cations and their counter-ions are re-dispersed randomly in the liquid electrolyte, further away from the AC surfaces (FIG. 1(B)). In other words, neither the cations nor the anions are exchanged between the anode surface and the cathode surface.            c. When in a charged state, a LIC also stores lithium in the interior of graphite anode particles (FIG. 1(E)) or Li4Ti5O12 particles (FIG. 1(F)), thus requiring a long re-charge time as well. During discharge, lithium ions must also diffuse out of the interior of graphite particles, thereby compromising the power density. The lithium ions (cations Li+) and their counter-ions (e.g. anions PF6−) are randomly dispersed in the liquid electrolyte when the LIC is in a discharged state (FIG. 1(F)). In contrast, the lithium ions are captured by graphene surfaces (e.g. at centers of benzene rings of a graphene sheet) when an SMC is in a discharged state. Lithium is deposited on the surface of an anode (anode current collector and/or anode active material) when the SMC is in a charged state. Relatively few lithium ions stay in the liquid electrolyte.            d. For a supercapacitor exhibiting a pseudo-capacitance or redox effect, either the cation or the anion form a redox pair with an electrode active material (e.g. polyanniline or manganese oxide coated on AC surfaces) when the supercapacitor is in a charged state. However, when the supercapacitor is discharged, both the cations and their counter-ions are re-dispersed randomly in the liquid electrolyte, away from the AC surfaces. Neither the cations nor the anions are exchanged between the anode surface and the cathode surface. In contrast, in a SMC, the cations (Li+) are captured by cathode surfaces (e.g. graphene benzene ring centers) when the SMC is in the discharged state. It is also the cations (Li+) that are captured by surfaces of an anode current collector and/or anode active material) when the SMC is in the discharged state. In other words, the lithium ions are shuttled between the anode surfaces and the cathode surfaces.            e. An SMC operates on the exchange of lithium ions between the surfaces of an anode (anode current collector and/or anode active material) and a cathode (cathode active material). The cathode in a SMC has (a) benzene ring centers on a graphene plane to capture and release lithium; (b) functional groups (e.g. attached at the edge or basal plane surfaces of a graphene sheet) that readily and reversibly form a redox reaction with a lithium ion from a lithium-containing electrolyte; and (c) surface defects to trap and release lithium during discharge and charge. Unless the cathode active material (e.g. graphene, CNT, or disordered carbon) is heavily functionalized, mechanism (b) does not significantly contribute to the lithium storage capacity.                        
When the SMC is discharged, lithium ions are released from the surfaces of an anode (surfaces of an anode current collector and/or surfaces of an anode active material, such as graphene). These lithium ions do not get randomly dispersed in the electrolyte. Instead, these lithium ions swim through liquid electrolyte and get captured by the surfaces of a cathode active material. These lithium ions are stored at the benzene ring centers, trapped at surface defects, or captured by surface/edge-borne functional groups. Very few lithium ions remain in the liquid electrolyte phase.
When the SMC is re-charged, massive lithium ions are released from the surfaces of a cathode active material having a high specific surface area. Under the influence of an electric field generated by an outside battery charger, lithium ions are driven to swim through liquid electrolyte and get captured by anode surfaces, or are simply electrochemically plated onto anode surfaces.                (B) In a discharged state of a SMC, a great amount of lithium atoms are captured on the massive surfaces of a cathode active material. These lithium ions in a discharged SMC are not dispersed or dissolved in the liquid electrolyte, and are not part of the electrolyte. Therefore, the solubility limit of lithium ions and/or their counter-ions does not become a limiting factor for the amount of lithium that can be captured at the cathode side. It is the specific surface area at the cathode that dictates the lithium storage capacity of an SMC provided there is a correspondingly large amount of available lithium atoms at the lithium source prior to the first discharge/charge.        (C) During the discharge of an SMC, lithium ions coming from the anode side through a separator only have to diffuse in the liquid electrolyte residing in the cathode to reach a surface/edge of a graphene plane. These lithium ions do not need to diffuse into or out of the volume (interior) of a solid particle. Since no diffusion-limited intercalation is involved at the cathode, this process is fast and can occur in seconds. Hence, this is a totally new class of energy storage device that exhibits unparalleled and unprecedented combined performance of an exceptional power density, high energy density, long and stable cycle life, and wide operating temperature range. This device has exceeded the best of both battery and supercapacitor worlds.        (D) In an f-SMC, the energy storage device operates on lithium ion exchange between the cathode and the anode. Both the cathode and the anode (not just the cathode) have a lithium-capturing or lithium-storing surface and both electrodes (not just the cathode) obviate the need to engage in solid-state diffusion. Both the anode and the cathode have large amounts of surface areas to allow lithium ions to deposit thereon simultaneously, enabling dramatically higher charge and discharge rates and higher power densities.        
The uniform dispersion of these surfaces of a nano-structured material (e.g. graphene, CNT, disordered carbon, nano-wire, and nano-fiber) at the anode also provides a more uniform electric field in the electrode in which lithium can more uniformly deposit without forming a dendrite. Such a nano-structure eliminates the potential formation of dendrites, which was the most serious problem in conventional lithium metal batteries (commonly used in 1980s and early 1990s before being replaced by lithium-ion batteries).                (E) A SMC typically has an open-circuit voltage of >1.0 volts (most typically >1.5 volts) and can operate up to 4.5 volts for lithium salt-based organic electrolyte. Using an identical electrolyte, a corresponding EDLC or symmetric supercapacitor has an open-circuit voltage of essentially 0 volts and can only operate up to 2.7 volts. Also using an identical electrolyte, a LIC operates between 2.2 volts and 3.8 volts. These are additional manifestations of the notion that the SMC is fundamentally different and patently distinct from both the EDLC and the LIC.        
The amount of lithium stored in the lithium source when a SMC is made dictates the amount of lithium ions that can be exchanged between an anode and a cathode. This, in turn, dictates the energy density of the SMC.
In these co-pending patent applications [Ref 1-6] we used graphene or disordered carbon as a cathode active material for a SMC cell, wherein the anode contains only a current collector or a current collector and an anode active material having high surfaces on which lithium can be electrochemically deposited. The anode active material (e.g. graphene) in a SMC does not involve lithium intercalation and de-intercalation.
In a co-pending application (G. Chen, et al, U.S. application Ser. No. 13/385,561 (Feb. 27, 2012)), isolated graphene sheets are used as a cathode active material for a lithium-ion cell (not SMC). In the instant application, an array of meso-porous graphitic materials (not including isolated graphene sheets) is used as the cathode active material. In both co-pending applications, the Li-ion cell contains a high-capacity anode active material (e.g. Si, Sn, or SnO2) and/or a high-rate capable anode active material (e.g. nano-scaled Mn3O4 particles). These anode active materials (e.g. Si, Sn, SnO2, and Mn3O4) in the lithium-ion cells operate on lithium intercalation and de-intercalation. These combinations lead to several unexpected yet highly significant results. Experimental evidence indicates that the electrochemical behaviors of these Li-ion cells and the SMC cells are vastly different and fundamentally distinct.
In summary, the current cathode active materials commonly used in Li-ion batteries have the following serious drawbacks:                (1) The practical capacity achievable with current cathode materials (e.g. lithium iron phosphate and lithium transition metal oxides) has been limited to the range of 150-250 mAh/g and, in most cases, less than 200 mAh/g.        (2) The production of these cathode active materials normally has to go through a high-temperature sintering procedure for a long duration of time, a tedious, energy-intensive, and difficult-to-control process.        (3) The insertion and extraction of lithium in and out of these commonly used cathodes rely upon extremely slow solid-state diffusion of Li in solid particles having very low diffusion coefficients (typically 10−8 to 10−14 cm2/s), leading to a very low power density (another long-standing problem of today's lithium-ion batteries).        (4) The current cathode materials are electrically and thermally insulating, not capable of effectively and efficiently transporting electrons and heat. The low electrical conductivity means high internal resistance and the necessity to add a large amount of conductive additives, effectively reducing the proportion of electrochemically active material in the cathode that already has a low capacity. The low thermal conductivity also implies a higher tendency to undergo thermal runaway, a major safety issue in lithium battery industry.        (5) The most commonly used cathodes, including lithium transition metal oxides and lithium iron phosphate, contain a high oxygen content that could assist in accelerating the thermal runaway and provide oxygen for electrolyte oxidation, increasing the danger of explosion or fire hazard. This is a serious problem that has hampered the widespread implementation of electric vehicles.        
Thus, it is an object of the present invention to provide a high-capacity cathode active material (preferably with a specific capacity greater than 250 mAh/g) for use in a lithium ion cell.
It is another object of the present invention to provide a high-capacity cathode active material exhibiting a specific capacity greater than 500 mAh/g, typically greater than 1,000 mAh/g, often greater than 2,000 mAh/g, or even greater than 3,000 mAh/g.
It is still another object of the present invention to provide a high-capacity cathode active material (with a specific capacity significantly greater than 250 mAh/g, up to 3,540 mAh/g) that can be readily prepared without going through an energy-intensive sintering process.
Another object of the present invention is to provide a high-capacity cathode active material (with a specific capacity greater than 250 mAh/g) that is amenable to being lithium intercalation-free or fast lithium intercalation, leading to a significantly improved power density.
Yet another object of the present invention is to provide a high-capacity cathode active material that is electrically and thermally conductive, enabling high-rate capability and effective heat dissipation.
It is still another object of the present invention to provide a high-capacity cathode active material that contains little or no oxygen, reducing or eliminating the potential fire hazard or explosion.
It is an ultimate object of the present invention to provide a high energy density lithium-ion cell that features a high-capacity cathode active material described above.